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Extinct Antarctic microbes raise thorny questions about the search for aliens

Even in the harshest environments, microbes always seem to outgrow each other. They thrive everywhere, from the hydrothermal forces from the bottom of the warm sea to the top of the mountain. Everest. Clusters of microbial cells clinging to the hull of the International Space Station were even found (SN: 26/08/20).

There was no reason for microbial ecologist Noah Fierer to expect the 204 soil samples he and his colleagues had collected near Antarctica’s Shackleton Glacier to be different. A typical tablespoon of soil could easily contain billions of microbes, and Antarctic soils in other regions hold at least a few thousand per gram. So he assumed that all of his samples would have at least some life, even though the air around the Shackleton Glacier is so cold and so arid that Fierer often left his wet clothes outside to freeze.

Surprisingly, some of the coldest, driest soils did not appear to be inhabited at all by microbes, according to him and his colleagues in the Journal of Geophysical Research: Biogeosciences. Let Fierer know, this is the first time scientists have found soils that don’t seem to support any kind of microbial life.

The findings suggest that extremely cold and arid conditions may put a hard limit on microbial habitability. The results also raise questions about how negative scientific results should be interpreted, especially in the search for life on other planets. "The challenge goes back to these kind of philosophical questions: how do you prove to be negative?" Proud of.

Noah Fierer and colleagues found soil samples from the Shackleton Glacier region of Antarctica that had no traces of life, an unexpected observation as samples from the continent typically contain thousands of microbes.Courtesy of N. Fierer

Testing a negative result is notoriously difficult. No measurement is perfectly sensitive, which means that there is always the possibility that a well-executed experiment may not detect something that is actually there. Years of experiments based on multiple independent methods passed before Fierer of the University of Colorado Boulder and his collaborator Nick Dragone felt safe enough to announce that they had found soils apparently free of microbes. And the scientists intentionally stated only that they were not able to detect life in their samples, not that the soils were naturally sterile. "We can't say the soils are sterile. No one can say that," Fierer says. "That's an endless search. There's always another method or a variant of a method you can try."

Polar microbiologist Jeff Bowman interprets the team's findings as false negatives. “Of course, there were things there,” says Bowman of the Scripps Institution of Oceanography in La Jolla, California. "This is the Earth. This is an environment that is massively polluted with life."

Even if there were some undetected microbes in the soil, Dragone said, that wouldn’t undermine his team’s evidence that cold and aridity pose a serious challenge to life. “It’s the combination of multiple very challenging environmental conditions that restricts life rather than an act alone,” Dragone says. "It's a very different kind of restriction than, say, high temperature."

As scientists look for evidence of life beyond Earth (SN: 28/07/20), they will inevitably be forced to walk the line between evidence of absence and the absence of evidence. “What we try to do on Mars is the opposite of what we try to do on Earth,” says polar microbiologist Lyle Whyte of McGill University in Montreal. On Earth, claiming that an environment has no life is a tough scientific sale. On Mars, it will be the other way around.

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